Sea Otters and Kelp Forests

In ecology, a keystone species is one that has a much larger effect on its environment than its numbers would suggest. The term comes from architecture: a keystone is the central stone at the top of an arch, and if it is removed, the whole arch falls down. One of the best-known examples of a keystone species is the sea otter of the North Pacific.

Sea otters feed on sea urchins, spiny animals that eat kelp, a large seaweed that forms underwater forests. Kelp forests provide food and shelter for many kinds of fish and other sea life. During the eighteenth and nineteenth centuries, however, sea otters were hunted so heavily for their fur that they disappeared from much of their range. Without otters to control them, urchin populations grew rapidly, and in many areas they ate the kelp until almost none was left.

In the 1970s, researchers compared islands in Alaska where otters had returned with nearby islands where otters were still absent. The difference was striking. Around islands with otters, the water was full of kelp and fish, while around islands without them, the sea floor was covered with urchins and little else. Since hunting was banned, otters have returned to many parts of the coast, and in several places the kelp forests have recovered.